The synthesis process to produce melamine starting from urea is usually carried out in presence of excess NH.sub.3, in order to limit the formation of deammonisation products such as melam-melem, and it can be globally expressed with the endothermic reaction:
6 CO (NH.sub.2).sub.2 +n NH.sub.3 =C.sub.3 N.sub.3 (NH.sub.2).sub.3 +3CO.sub.2 +6 NH.sub.3 +n NH.sub.3 From the chemistry of the process it can be seen that for every mole of melamine obtained, three moles of CO.sub.2 and six moles of NH.sub.3 together with the nNH.sub.3 introduced in excess, are generated.
The endothermic character of the reaction, industrially carried out at temperatures in the range of 400 C..degree., requires the supply of 649 KJ per mole of melamine (starting from molten urea at 135 C..degree.) usually realized by heat transfer from molten salts circulating in coils or bayonet tubes situated in the reaction zone.
At level of industrial application, the production processes of melamine from urea are usually classified in:
catalytic processes at low-pressure: p.ltoreq.1 MPa PA1 non-catalytic processes at high-pressure; p.gtoreq.5 MPa PA1 reaction of the urea to melamine PA1 separation of the residual gas/ageing of the melamine PA1 washing of the residual gas PA1 the top sector, where the molten urea is supplied and the reaction gas is washed, is provided with at least one nozzle for the molten urea inlet, with at least one distributor for the reaction gas connected to a duct communicating with the central sector through a hole in the diaphragm, with at least one pipe connecting said sector with the lower sector of reaction, through which the molten urea reaches said reaction sector, with at least one valve for extracting the residual gas and for adjusting the working pressure of the reactor, with a heat exchange system including at least one heat exchanger cooling the urea heated by the reaction gases; PA1 the bottom sector, wherein the chemical reaction of the melamine synthesis occurs in the presence of an excess of ammonia, is equipped with at least one element for the distribution of the gaseous ammonia, said element being connected to an external pipeline, with at least one essentially vertical pipe for feeding the molten urea coming from the top sector, with at least one heat-exchanger for obtaining the heat energy necessary for the endothermic reaction, with at least one opening made in the diaphragm separating said bottom sector from the central sector and feeding the reaction product into the central sector; PA1 the central sector, wherein the separation of the gaseous products from the liquid phase of the reaction occurs, is supplied with at least one pipe for feeding the reaction products coming from the bottom sector, with at least one distributor of the gaseous ammonia, with means for keeping the temperature of said sector under control, with at least one hole made in the diaphragm, separating said sector from the top sector and suited for transferring the reaction gases to the top sector, with at least one tube for the outlet of the melamine produced, said sector being crossed by at least one pipeline connecting the top sector with the bottom sector, PA1 each of said sectors presenting in correspondence with its bottom, drain nozzles for recovering the liquids and for the maintenance of said sectors and means for measuring the temperature.
Reference is made hereinafter to the non-catalytic processes at high-pressure and especially to the process steps required to obtain raw molten melamine and scrubbed reaction off-gas.
The degree of purity of the raw melamine depends on the choices of a series of process parameters such as temperature, pressure, residence time and excess of ammonia, as described for instance in the U.S. Pat. No. 3,484,440 and it is generally over 95%. Following treatments of purification and crystallisation permit to reach purity degrees of .gtoreq.99.9% as described for instance in the U.S. Pat. No. 3,454,571.
Also the composition of the raw off-gas produced by the reaction, depends on the choice of the above mentioned process parameters.
Such gases, separated from the raw melamine in the liquid phase, contain considerable amounts of melamine, urea and other byproducts as described, for instance, in the U.S. Pat. No. 3,700,672 and quantified in the tables 1 and 2 of the same.
Before recycling them to the urea-plant or before they undergo the process for the recovery of the ammonia they contain, they need to be properly washed for the recovery of the melamine and the urea and for the maximum possible energy recovery.
At present and generally, the high pressure non-catalytic processes, industrially used, start from pressurized molten urea at temperatures comprised between 135.degree. and 160.degree. C., and operate in a pressure range of 5 to 20 MPa and temperature range of 370.degree. to 430.degree. C.
From the point of view of the process design, the plant engineering, equipment and plant operation, the synthesis section of a melamine plant can be schematically subdivided into three steps:
which, with considerable differences in the equipment design and process operating conditions, are performed in specific equipment separated from one another, as is the case in the processes adopted from Allied Chemical, Nissan, MCI and Montecatini (Ausind).
(Ulmann's Encyclopedia of Industrial Chemistry--5th edition)
(Nitrogen N.degree. 139 Sept./Oct. 1982)
The high-pressure process for producing melamine in the liquid phase has also the advantage of yielding the residual gas at high pressure which can be easily recycled in the urea synthesis plant.
It also lends itself, in comparison with the low-pressure catalytic process, to smaller vessels, piping and equipment design but the corrosive behaviour of the processing fluids dictates the use of costly and sophisticated corrosion resistant materials, such as "Hastelloy" and titanium. If, as it happens at present, the mentioned steps of the process are realized industrially step by step in specific equipment, then it is necessary to provide the plant with complex pipeline systems for conveying the processing fluids, with block and control valves, purges and drains, washing systems, jacketing, specific instrumentation, etc. . . . which make the plant complex, difficult to operate, to start-up and to shut-down.
The presence of so many high-pressure pipelines, valves, flanges and fittings, increases, moreover, the risk of plugging of leakages and corrosion of the materials and it affects negatively the safety and the reliability of the plant, the respect of the ecological rules and the quality control of the product.
Such step by step process and plant design approach led to the introduction of specific and itemized improvements of the process and of the equipment design, such those described for instance, in the already mentioned U.S. Pat. Nos. 3,454,571; 3,484,440; 3,700,672.
This implies a complex and sophisticated plant lay-out requiring a series of ancillary equipment and auxiliary utilities with a consequent increase in the investment costs and a probable decrease in the plant utilization factor.